The heart is the first major organ to function during development yet w4e know almost nothing regarding the regulation. It has been established that conditions encountered during that conditions encountered during development can affect the function of the adult. It is also known appropriate of the heart and circulation requires the pulsatile pumping activity of the heart. In adults the cardiovascular system is regulated by complex neural and hormonal systems which are absent in embryos. Despite the well-documented lack of neural control early in development, preliminary studies on Xenopus laevis larvae convincingly show the ability to correct increased arterial blood pressure induced by volume load. This project will continue these investigations to define the regulatory mechanisms that make this control possible. One important factor that regulates cardiac function is preload. It is the goal of this research project to define the factors that determine preload including blood volume, vascular blood volume, vascular compliance, and local regulatory compounds including adrenergic transmitters and nitric oxide. By using servo-null micropressure techniques in combination with video image analysis, it is possible to evaluate cardiac function in animals weighing under 100 mg. By volume loading these animals and evaluating their ability to return blood pressure to normal with and without functioning local control will lead to further understanding of the abilities of early embryos to regulate their own developmental paths and perhaps to defend themselves against transient interruptions of the normal course of development.